Hitherto, an organic solvent based ink for a ball point pen or a marker pen generally contains an organic solvent and a solvent dye which is listed in color index. The organic solvent has been changed from an aromatic solvent to an alcohol and a glycohol because of toxicity, air pollution and the like. However, only a few solvent dyes have high solubility with the alcohol and glycol and it is therefore very difficult to formulate various kind of inks.
There also proposed three kinds of dye, i.e. a dye which is color-developed by adding an organic acid to a leuco base of a basic dye, a dye which is prepared by salt-forming a base of the basic dye with an acidic dye and a dye which is prepared by forming an amine salt from the acid dye. They, however, do not have sufficient solubility in the alcohol and glycol, especially propyl alchol and propylene glycol which are less toxic. They are also poor in stability with time in the alcohol and glycol.
An organic solvent based ink which employs a pigment as colorant is also known to the art. Since the pigment inherently is not compatible with the organic solvent or a binder resin, the pigment must be ground with the binder resin or the solvent and dispersed in the ink. The pigmented ink, however, does not have stability with time and often forms agglomerates and precipitates.